1. Field of the Invention
The present invention relates to driving circuits and driving methods for liquid crystal display devices, and in particular to multiple line inversion driving in active matrix liquid crystal display devices.
2. Description of the Related Art
Active matrix liquid crystal display devices provided with TFTs (thin film transistors) as switching elements have been known for several years. Such liquid crystal display devices are provided with a liquid crystal panel which includes two insulating substrates that are arranged opposite one another. On one substrate of the liquid crystal panel, scanning signal lines and video signal lines are arranged in a lattice, and TFTs are arranged near the intersections of the scanning signal lines and the video signal lines. Each of the TFTs has a drain electrode, a gate electrode branching off from the scanning signal lines, and a source electrode branching off from the video signal lines. The drain electrodes are connected to pixel electrodes that are arranged in a matrix on the substrate for forming an image. Also, the substrate on the other side of the liquid crystal panel is provided with an opposing electrode for applying a voltage between the pixel electrodes and the opposing electrode, across the liquid crystal layer. The individual pixels are formed by the pixel electrodes, the opposing electrode and the liquid crystal layer. It should be noted that, for the sake of convenience, regions forming single pixels are referred to as “pixel formation portions”. Moreover, a voltage is applied to the pixel formation portions based on a video signal that the source electrodes of the TFTs receive from the video signal lines when the gate electrodes of the TFTs receive an active scanning signal from the scanning signal lines. At each of the pixel formation portions, a pixel capacitance is formed by the pixel electrode and the opposing electrode, and the pixel capacitance holds a voltage indicating the pixel value.
Now, the liquid crystal has the property of degrading when a DC voltage is applied to it continuously. Therefore, an AC voltage is applied to the liquid crystal layer in the liquid crystal display device. This application of the AC voltage to the liquid crystal layer can be realized by inverting the polarity of the voltage applied to each of the pixel formation portions in every single frame period, that is, by inverting in every single frame period the polarity of the voltage of the source electrode (video signal voltage) when taking the voltage of the opposing electrode as the reference. As technologies for realizing this, a driving method known as line inversion driving and a driving method known as dot inversion driving are known. It should be noted that in the following, the voltage applied to the pixel formation portions is referred to as “pixel voltage”.
In line inversion driving, the polarity of the pixel voltage is inverted in every single frame period and at every predetermined number of signal scanning lines. For example, a driving method in which the polarity of the pixel voltage is inverted in every single frame period and at every two scanning signal lines is referred to as “2-line inversion driving”. On the other hand, in dot inversion driving, the polarity of the pixel voltage is inverted in every single frame period, and also the polarities of pixels that are adjacent in the horizontal direction are inverted within a single frame period.
FIGS. 10A to 10C are polarity diagrams showing the polarities of the pixel voltages applied to the pixel formation portions on the display screen for a given frame period in a conventional liquid crystal display device. It should be noted that FIGS. 10A-C show the polarities only for a portion (four rows×four columns) of the display screen. FIG. 10A shows the polarities for the case of 1-line inversion driving. As shown in FIG. 10A, in the direction in which the scanning signal lines extend, the polarities of all of the pixel formation portions are the same. On the other hand, in the direction in which the video signal lines extend, the polarities of the pixel formation portions are inverted at every single pixel formation portion.
It is difficult to make the transmittance of the liquid crystal when the polarity of the pixel voltage is positive the same as the transmittance of the liquid crystal when the polarity of the pixel voltage is negative. The reason for this is, for example, that the on-current of the TFT differs depending on whether the polarity of the pixel voltage is positive or negative. For this reason, a pattern of horizontal lines tends to be perceivable in the case of the above-described 1-line inversion driving, for example when a uniform luminance is displayed on the entire display screen.
FIG. 10B shows the polarities for the case of dot inversion driving. As shown in FIG. 10B, in dot inversion driving, the polarities of the pixel voltages are inverted at all neighboring pixels, so that the above-noted problem does not occur. However, with conventional dot inversion driving, the polarity of the pixel voltages is inverted at every single scanning signal line, so that there is the problem that the power consumption is large.
In order to solve this problem, JP H08-43795A discloses a liquid crystal display device, in which the polarities of the pixel voltages are inverted at every two scanning signal lines and the polarities are also inverted between pixels adjacent in the horizontal direction. FIG. 10C shows the polarities of the pixel voltages of this liquid crystal device. With this liquid crystal device, the polarities are inverted between pixels adjacent in the horizontal direction, thus solving the problem that occurs in the case of line inversion driving. Moreover, the polarities of the pixel voltages are inverted at every two scanning signal lines, so that the power consumption is lower than in the case of inverting at every signal scanning signal line. It should be noted that the driving method of this liquid crystal display device is also referred to as “2-line dot inversion driving”.
In recent years, however, the resolution of liquid crystal display devices is becoming increasingly higher, and the number of scanning signal lines in the device is steadily increasing. Therefore, the length of the horizontal scanning period is becoming shorter, and the time for accumulating charges in the pixel capacitances (charge time) may not be sufficient. Moreover, as liquid crystal display devices become larger, also the rise time until the voltage of the source electrodes of the TFTs have reached a target voltage with the video signals becomes longer. FIGS. 11A to 11E are signal waveform diagrams for the case of the above-described 2-line dot inversion driving. FIG. 11A shows the signal waveform of the video signal S(k) of the k-th column. FIG. 11B shows the signal waveform of the scanning signal G(j) of the j-th row. FIG. 11C shows the signal waveform of the scanning signal G(j+1) of the (j+1)-th row. FIG. 11D shows the signal waveform of the scanning signal G(j+2) of the (j+2)-th row. FIG. 11E shows the signal waveform of the scanning signal G(j+3) of the (j+3)-th row. T1 through T4 each denote one horizontal scanning period. As shown in FIGS. 11B to 11E, the scanning signals are successively made active in the direction in which the video signal lines extend. Also, the time of the active state (pulse width) is the same for all scanning signals G(j) to G(j+3). In this case, for the reasons explained above, a sufficient charge may not be accumulated in the pixel capacitances of the pixel formation portions to which a video signal S(k) is supplied whose polarity is inverted from the previous horizontal scanning period, as in the periods T1 or T3, so that only a pixel potential that is lower than the desired gray-scale potential is attained. On the other hand, in the pixel formation portions to which a video signal S(k) is supplied whose polarity is the same as in the previous horizontal scanning period, as in the periods T2 or T4, the signal voltage is already at a sufficiently high potential, so that sufficient charges accumulate in the pixel capacitances. Therefore, the charge amount accumulated in the pixel capacitances differs between the pixel formation portions to which a video signal is supplied whose polarity is inverted from the previous horizontal scanning period and the pixel formation portions to which a video signal is supplied whose polarity is the same as in the previous horizontal scanning period, which may cause a decrease in display quality. For example, when displaying a uniform luminance on the entire screen, a pattern of horizontal lines appears on the screen. It should be noted that in the following, the ratio of the actual pixel potential of a given pixel formation portion to the desired gray-scale potential of that pixel formation portion is referred to as “charge ratio.”